1. Field of the Invention
This invention relates to optical waveguide fibers, and, in particular, to an improved heating oven for use in preparing such fibers.
2. Description of the Prior Art
Various conventional techniques for preparing optical waveguide fibers involve the use of heating ovens capable of generating temperatures in the range of from about 1000.degree. C. to about 1500.degree. C.
For example, in the outside vapor deposition (OVD) process, a porous glass preform, also referred to as a soot blank, is formed and then placed in a heating oven for drying and consolidation into a transparent glass blank suitable for drawing into an optical waveguide fiber. Drying is accomplished by heating the preform to a temperature of about 1100.degree. C. in the presence of one or more drying gases, such as, a mixture of helium and chlorine. Consolidation is accomplished by heating the dried preform above its sintering temperature, e.g., to a temperature on the order of 1450.degree. C.
In the past, the heating ovens used for drying and consolidation have employed a stationary "hot zone" and a moving blank. More specifically, the ovens have included a cylindrically-shaped muffle, e.g., an alumina muffle, having a length of about 3 meters and an inside diameter of about 12.7 cm. The outside of the muffle has been insulated with, for example, an alumina insulation to minimize heat loss. A heating element, e.g., a resistance heating element, has been placed around the center of the muffle producing a central hot zone, having a length of approximately 26 cm, surrounded on either side by cooler end zones, having lengths of approximately 84 to 99 cm.
Drying has been accomplished within these ovens by introducing the soot preform into the upper section of the muffle and introducing drying gases into the muffle. In some cases, the soot blank has been rotated about its long axis during the drying process. Typically, the soot blank has been kept in the upper section of the muffle for approximately 3 to 4 minutes prior to consolidation.
Consolidation has been performed within these ovens by lowering the soot blank into the hot zone at a rate of approximately 5 mm per minute while the temperature of the hot zone was held at approximately 1500.degree. C. During consolidation, the soot blank may or may not have been rotated, as desired.
These prior art heating ovens have suffered from various disadvantages, the most limiting of which has been the fact that the ovens have been commercially practical only when used to process short blanks, e.g., blanks having a length on the order of 75 cm.
To accommodate both the central hot zone and the cooler end zones, the prior art ovens have required oven muffles having a length on the order of about 3-4 times the length of the longest blank to be processed in the oven. Accordingly, for a 75 cm blank, a 3 meter muffle has been required. Because muffle pieces longer than 1.8 meters have not been available in commercial quantities, three meter muffles have typically been constructed by joining together two muffle pieces, with the joint thereafter being exposed to temperatures no greater than 1100.degree. C., that is, the joints have been kept out of the hot zone.
For a 2 meter blank, as opposed to a 75 cm blank, muffles having a length on the order of 6 meters would be required. Although it is possible to fabricate a 6 meter muffle by joining together three or more muffle pieces, the joining and aligning process is difficult to accomplish, especially in view of the fact that the joint must be gas tight so as to retain the processing gases within the muffle. Also, there is the potential for failure of the muffle at the two or more joints due to the effects of the processing gases and the elevated temperatures employed in drying and consolidating blanks.
In addition to the muffle problems, the application of the prior art heating ovens to the processing of long blanks has raised various other problems. For example, the physical space needed to house a prior art oven capable of processing long blanks, including the oven's muffle, its supporting structure, and the equipment used to insert and remove blanks from the muffle, has quickly become prohibitive with increasing blank size. Thus, for a 75 cm blank, an oven and its associated equipment can be housed in a two-story building. For a 2 meter blank, on the other hand, a three-story building would be needed. This plainly makes for an expensive oven.
Moreover, as the overall length of the oven and its associated apparatus is increased, it becomes substantially more difficult to maintain the requisite control over the movements of the blank. In particular, it becomes difficult to align three stories of apparatus so that a 2 meter blank, can be both rotated and translated along the centerline of a 6 meter muffle.
In addition to their physical limitations, the prior art ovens have also suffered from processing limitations. In particular, due to the stationary hot zone and moving blank, only simple thermal cycles of the types described above for drying and consolidation have been conveniently performed within these ovens.